Target comprising thickness profiling for an RF magnetron

ABSTRACT

Method for sputtering from a dielectric target ( 9 ) in a vacuum chamber ( 2 ) with a high frequency gas discharge, the target ( 9 ) being mounted on a cooled metallic back plate ( 10 ) and this back plate forming an electrode ( 10 ) supplied with high frequency, includes a target thickness (Td) profiled ( 15 ) differently over the surface such that in the regions of a desired decrease of the sputtering rate the target thickness (Td) is selected to be greater than in the remaining regions.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to a method for sputtering a dielectrictarget in a vacuum chamber with a high frequency gas discharge, as wella sputter target and a magnetron sputter source with a correspondingsputter target. The dielectric layer is deposited onto a workpiece, inparticular a synthetic workpiece, with the aid of the known highfrequency cathode sputtering method, in particular with magnetronsputtering in a vacuum chamber. Such layers are in particular applied inthe production of storage plates. Among them for example opticalrecording methods, where the information is impressed in the plateitself and is provided with a highly reflecting layer, with a laser beambeing capable of scanning the information correspondingly. In particularin the vacuum coating of optical disks with the aid of the staticsputtering method with round cathode configurations with nonconductingmaterials, and especially in the coating of rewritable disks, good layerthickness distributions over the disk are necessary and must also bemaintained over the process time, respectively the target working life.

For coating optical storage disks metallic as well as also nonconductingdielectric layers are deposited on a substrate, the data medium, theconducting metallic layers can be coated relatively simply. For coatingwith dielectric layers two methods are in the foreground. The one methodis the so-called reactive sputtering from a conducting target. Thesputtered material is herein oxidized to a nonconducting layer in theprocess volume or when impinging on the substrate with the aid of thereactive gas introduced into the process volume.

The other method for sputtering nonconducting substrate material is thehigh frequency nebulizing sputtering. Since in this case the target isnot conducting, it cannot be sputtered with a DC voltage, since throughthe formation of a surface charge, the entire applied voltage drops overthe target and not, as desired, in the plasma volume. Consequently nocurrent can flow through the target into the plasma volume. By applyinga radio frequency AC voltage the surface charges can each be led duringthe positive half wave from the target, the target acts like a capacitorwith impedance Z =1/iωC and causes a dielectric displacement current toflow. The frequencies required for this purpose are in the highfrequency range, that is in the range of >1 MHz, and the industrialfrequency in the range of 13 MHz for practical reasons is a suitablechoice.

To increase the target utilization and to improve the layer uniformityon the substrate, the target is eroded over a relatively large radialarea. A close relationship exists between erosion profile and layerthickness distribution on the substrate. The desired erosion profile canbe generated for example through a rotating magnet system, throughsuitably disposed magnets and pole shoes or through correction effects,for example additionally over the target working life or temporarilyvariable magnetic fields. Especially in statically disposed substrates,such as the storage disks, which are stationarily disposed at a distanceof a few centimeters opposite a flat target, these conditions play aspecial role. In such vacuum coating systems the disks are cycled inthrough an interlock and coated in front of the target at a defineddistance, with the coating time as a rule amounting to a few seconds toa few minutes. With such a sputter target thousands of disks are coateduntil it must be replaced after the target has been eroded so far, orwhen the erosion profile is too deep and the target must be replaced.Through the erosion profile, which changes over the working life of thetarget, the distribution conditions on the substrate also change, andthis can have negative effects on the distribution especially inmagnetron sputtering sources which have especially high sputtering ratesand therefore are especially preferred today.

To generate a suitable erosion profile during DC sputtering, variousmethods are applied. For example, through the suitable disposition ofthe magnets which rotate in the region of the backside of the target,the most diverse erosion profiles can be generated. With the suitablechoice of magnet configuration, the profile of the erosion rate can alsobe kept substantially constant even over the entire target working life.But it was found that this cannot be carried out in the same manner inhigh frequency sputtering (RF sputtering). For this reason attempts aremade for example to compensate the discrepancies over the target workinglife with additional control magnets or with multi-part configuration ofthe cathode through suitable driving. These methods have been known fora relatively long time but they have the disadvantage that they areexpensive in realization and that highly complex conditions obtain. Thecomplexity of the processes increases the probability of operatingerrors, especially in industrial use.

SUMMARY OF THE INVENTION

The present invention addresses the problem of eliminating thedisadvantages of prior art, especially in realizing a coating method fordielectric materials with which at high economy a predeterminabledistribution profile can be attained, and which in addition provides thecapability of compensating distribution changes over the target workinglife of a sputter source. This problem is solved according to theinvention after the method according to the independent claims. Theindependent patent claims refer to advantageous further embodiments.

The dielectric, and consequently nonconducting, sputter target materialis secured, or bonded, for high frequency sputtering on a so-calledsupport plate or back plate, and this must be completed in such a mannerthat good thermal contact is given. The support plate is necessary withdielectric materials, on the one hand, to hold the brittle material and,on the other hand, to attain good heat distribution. The support plateis cooled and consequently indirectly also the target bonded thereon.

As stated, an important application is the deposition of dielectriclayers for optical storage plates, such as in particular for phasechange disks. An important application herein is the coating with atarget material comprising ZnS and SiO₂. As a rule, such targets aresintered during their production. Maintaining maximum layer uniformityduring the process times is especially important in this application.

It was found that the capacitance developing between metallic conductingbonding plate and the plasma developing in the front region of thetarget is different if the dielectric target is developed of differentthickness with respect to the bond plate in subregions of the target. Itthereby becomes possible to affect locally the discharge or thedischarge density. Through this approach the sputter distribution can beaffected if in the desired regions the target thickness is varied overthe target face. In the thinner region the capacitance between plasmaand target support plate is increased, which also results in theseregions in an increase of the sputter rate. Through the correspondingprofiling of the target, consequently, the distribution can be affectedto the desired extent. This type of correction capability is especiallyfavorable when using magnetron sputter sources, which develop inherentlydifferent erosion profiles through the magnetic field-enhanced plasmageneration and these erosion profiles, generated by the magnetic field,effect distribution profile shifts over the target working life. Forexample, in regions of strong erosion due to the magnetic field thispronounced erosion can be compensated by a thickening of the target viathe capacitive effect. In regions of the weaker erosion by the magneticfield through a thinning of the target an increase of the sputter rateis brought about. Consequently, a compensation of the effect is alsopossible. Profiling of the target can take place for example on thefront side facing the plasma, and the target support plate can be shapedon the back side as a planar plate. A further option also comprisesprofiling the support plate and to dispose between a flat target plateand the profiled support plate a further dielectric with good thermalconductivity, which is more readily workable than the target materialitself and consequently represents a compensation dielectric with theadvantage that the brittle target material itself can be implemented assimple as possible for example as a planar plate. The previously citedprofiling options can also be combined. But for reasons of simplemanufacture those solutions are preferred which permit the simplestpossible contours or even permit a purely plate-form target, wherewiththe production process can be carried out economically. Apart fromsimple depressions or elevations, other forms of profiling also have aneffect such as for example trapezoidal, spherical, toroidal ribs orgrooves. With the profiling thus the erosion rate, but in particular theradial distribution of the erosion rate in round targets, can bestabilized over the entire target working life. When sputteringconductive target materials with RF or DC sputtering, this effect doesnot occur.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be explained by example in furtherdetail in conjunction with schematic Figures. Therein depict:

FIG. 1 schematically and in cross section a basic configuration of avacuum installation with high frequency sputtering device,

FIG. 2 a target bonded on a support plate with erosion profile accordingto prior art,

FIG. 3 a target with erosion profile bonded on a support plate in rounddevelopment with profiling in the center and on the front side,

FIG. 4 in cross section a profiled target bonded on a support plate withprofiling in the center on the target back side,

FIG. 5 in cross section a planar target with erosion profile bonded on aprofiled support plate with compensation dielectric as intermediatelayer,

FIG. 6 a graph representing the erosion rate as a function of the targetworking life measured in different radii in the case of a flat roundtarget according to prior art,

FIG. 7 a graph showing the erosion rate as a function of the targetworking life, measured at different radii of a profiled round targetaccording to the invention,

FIG. 8 an electric equivalent diagram regarding the functional mechanismof a profiled dielectric target in a high frequency discharge,

FIG. 9 different distributions measured on a disk coated with RFsputtering over the diameter at different stages of the target workinglife for a planar target according to prior art,

FIG. 10 distribution measurement corresponding to the preceding FIG. 9for a target profiled according to the invention,

FIG. 11 cross section through a target at different target service livesfor a planar target according to prior art,

FIG. 12 cross section through a target profiled according to theinvention with different erosion profiles at different target workinglives.

DESCRIPTION OF THE PREFERRED EMBODIMENTS.

High frequency sputtering sources for vacuum coating processes areemployed in varied manner. Especially the magnetron sputtering techniquewith magnetic field-enhanced plasma generation is preferably employeddue to the high attainable rates and due to the capability of affectingthe sputtering characteristic via the magnetic field. Sputtering sourcesof this type can be developed in different geometries, for exampletubular with target tubes also rotating, planar with targets disposed areally, such as rectangular cathodes, or also round cathodeconfigurations, or also as cathodes developed in the form of an arch orthose developed hollow. In order to attain a good distribution of thedeposited layer, the workpieces or the substrates to be coated can bemoved with respect to the sputtering target. But they are often used forfully automated installations, so-called static coating configurations,in which the workpiece is stationarily disposed at a distance of a fewcentimeters opposite a cathode with sputtering target, which makes theinstallations become more compact and simpler. A typical stationarycondition is shown schematically in FIG. 1. A vacuum chamber (2) isevacuated with a vacuum pump (5) via a valve (6). The gases necessaryfor the generation of the plasma can be introduced via the gas inletarrangement (7, 8) and an appropriate working pressure is set. Asworking gases for the sputtering primarily heavy inert gases such asargon are suitable and in reactive processes additionally reactivegases, such as for example oxygen, nitrogen, etc. The vacuum chamber (2)includes a substrate holder (3), which here, as shown schematically, isdeveloped as a type of interlock, in that it can be lowered in order tobe able to equip the substrate holder (3) with a workpiece (4). It isunderstood that highly varied capabilities exist for developing suchloading stations. As a rule, separate interlock configurations areemployed such that the process volume is separated from theinward-transfer chamber and the vacuum conditions in the process area,in which is also disposed a sputtering source (1), are separated.Opposite the workpiece (4) at a distance (d) of, as a rule, a fewcentimeters, such as for example 4 to 5 cm, the sputtering source (1) isdisposed, which includes the target (9) to be sputtered. Between thetarget (9) and the substrate (4), or substrate holder (3), is suppliedpower at high frequency by means of high frequency generator (20) forgenerating a plasma discharge between target (9) and substrate (4). Inthe case of this type of sputtering processes, the vacuum chamber (2) iscustomarily grounded, and the substrate-side high frequency terminalalso, as is shown in dashed lines in the Figure. But it is also possiblein known manner to supply the sputtering source (1) as well as theworkpiece and/or also the receptacle with superimposed voltages withso-called bias voltages, to attain specific effects.

The sputtering source (1) includes the target (9) which is shown in FIG.2 in cross section. The Figure shows a typical structure of a targetconfiguration (9, 10), such as is used in the preferred magnetronsputtering sources, according to prior art. Herein a planar dielectrictarget plate (9) with a level new target face (13) is bonded onto asupport plate (10) which is cooled in a manner not shown here. On theback side of the target configuration (9, 10) a magnet system (11) isdisposed, which forms the electron trap configuration of the plasmadischarge. In such a magnetron sputtering source with round targetconfiguration (9, 10) the magnet system (11) in operation is preferablymoved about the central axis (12) in order to generate a suitable formof the erosion on the target surface through the eccentric dispositionof the magnetic field configuration. Similar techniques are alsoutilized for example in rectangular target configurations through linearmovements. Furthermore is shown an erosion profile (14), which has atypical development for a magnetron sputtering source, with the erosionprofile already showing an essentially consumed, or sputtered, target.

In FIG. 3 is again shown a cross section through a round target as inFIG. 2 with a sputtering target (9), which on the front side comprisesin the center a target profiling (15). The profiling (15) is developedin the form of steps, such that in the center the target thickness isdecreased and there according to the invention the rate is increasedthrough the capacitive effects of the high frequency discharge in orderto compensate in this area the effect of the strong erosion on the outermargin.

In a further embodiment in FIG. 4 is shown the manner in which a target(9) can be profiled on its back side, and the metallic support plate(10) is fitted into this depression of the profiling (16) in order toattain the inventive effect.

A further capability for realizing different dielectric thicknesses onthe target is shown in FIG. 5. Starting with a planar target (9), whichis especially simple and consequently economical to produce, theprofiling (18) is generated in the support plate (10) and between thetarget (9) and the support plate (10) is provided a further dielectriclayer (17) which acts as a compensation. The advantage of thisembodiment comprises that for the compensation dielectric (17) amaterial can be selected which can be formed more simply than thesputtering target material (9) and consequently the costs for the targetproduction can be substantially lowered. As the compensation dielectric(17) are suitable materials with an ε between 2 and 50, for examplesynthetic materials or also ceramic materials, with good heatconductance, such as for example aluminum oxide Al₂O₃. A significantadvantage in this configuration comprises furthermore that thecompensation dielectric (17) can be left on the support plate (10), andthe planar new target (9) can be adhered directly onto the planarinterface without having to generate the profiling itself anew eachtime.

In FIG. 6 for a planar sputtering target according to prior art and theembodiment according to FIG. 2 the manner is shown in which the erosionrate ER behaves as a function of the sputtering energy SE (targetworking life) in kWh, measured at the three erosion zones with radiusr0, r36 and r72 measured in mm from the center of the round target. Itis clearly evident that the erosion rates diverge from one anotherwhich, over the target working life leads to a shift of the distributionprofile on the substrate (see FIG. 9).

In FIG. 7 in the same representation the behavior of an inventive targetembodiment according to FIG. 2 is shown. It is herein evident that theerosion rates extend uniformly over the target working life, which leadsto the stabilization of the distribution profile on the substrate.Measured were the conditions in both cases with a 6 mm thick target ofZnS and SiO₂ with a diameter of 200 mm and a target substrate distanceof 40 mm, where the substrate diameter was 120 mm and the frequency ofthe RF generator was 13 MHz. To confirm the effect, with the samesputtering configuration also a flat aluminum target with identicaldimensions was sputtered. The tests show that the effect occursspecifically during high frequency sputtering of nonconductingmaterials, however not during high frequency sputtering of conductingmaterials.

For the qualitative description of the effect a highly simplifiedelectric circuit diagram of an RF sputtering configuration withinsulating target material is depicted in FIG. 8. Depicted is the radialsputter erosion rate through specific variable radius-dependent plasmaand target impedances Z_(p)(r) and Z_(t)(r). In addition to the targetcapacitance, C_(t) also the losses in the target, determined by the lossangle δ of the dielectric target, are of significance. The tangent ofthe loss angle is defined as tan(δ)=Im(Z)/Re(Z) of the impedance of thetarget measured at the working frequency of typically 13 MHz anddescribes the discrepancy from the purely capacitive behavior andconsequently also the energy losses in the dielectric. Values below 0.05for tan (δ) are attainable in commercially available targets. Typicalvalues for the total target capacitance are C_(t)=200 pF and the targetimpedance |Z|=60 Ω, for the plasma capacitance C_(p)=300 pF and theplasma impedance Im(Zt)=40 Ω. The real part of the impedanceR_(p)=R_(e)(Z_(p)) is approximately 20 Ω. The orders of magnitude of thevalues are comparable, especially the target capacitance affectssignificantly the plasma discharge. With increasing erosion the targetbecomes thinner and the target capacitance C_(t)=Aε/d becomes greater.Thereby the power distribution over the target and plasma is changed,the losses in the supply lines, shown schematically by Z_(L) (includingsupply impedance), are reduced and the rate increases with constantpower driving, as is evident in FIG. 7. Decisive for the powerdistribution are the differential impedances dU/dI in the plasma, whichare very low due to the flat characteristic in the voltage range used.If the thickness change of the target takes place selectively, forexample preferably on an outer radius of the target, the rate at thissite increases superproportionally since here the current densityincreases due to the decreasing target impedance. To prevent this,through a thickness change of the target an incremental impedance isadded in series with |Z| proportional to the thickness, which reducesthe acceleration of the erosion rate. The same effect is attainedthrough a reduction of the target thickness at the sites of deepererosion rate.

The positive effect of the invention will be once again described basedon the distribution measurements on the substrate. A planar dielectrictarget according to prior art was sputtered under the conditions asstated already and the distribution characteristic over the diskdiameter is measured at different target serving lives. The results areevident in FIG. 9, where the relative layer thickness S is depicted inpercentages as a function of the substrate position, or distance fromthe substrate center in mm, at different operating times t11 to t15 overthe target working life. For typical optical storage plates, only theregions from ±25 to 60 mm were measured, since there is a hole in thecenter of the synthetic disk. Curve t11 was measured at the beginning ofthe target working life, the curve t12 after 80 kWh, the curve t13 after200 kWh, the curve t14 after 270 kWh and the curve t15 after 385 kWh,that is approximately at the end of the working life of the target. Thedepiction shows that over the target working life strong shifts andtiltings of the distribution curves occur, wherewith the layer thicknessdistribution over the target working life on the storage plate varies inimpermissible manner. The associated target is depicted in cross sectionin FIG. 11. Shown are the different erosion profiles over the targetworking life, that is the target thickness in percent as a function ofthe target radius R in mm. The target surface not yet sputtered off isshown with profile e0, e1 shows the erosion profile which develops after80 kWh of operating time, e2 after 200 kWh operating time and e3 after385 kWh operating time, that is approximately at the end of the targetworking life.

For comparison, the same measurements under identical conditions with aninventive profiled target, corresponding to a target profiling accordingto FIG. 3, is shown in the graphs of FIGS. 10 and 12. T11 shows thedistribution at the beginning of the target working life, T12 after 60kWh, T13 after 121 kWh, T14 after 253 kWh and T15 after 307 kWh at theend of the target working life. FIG. 12 shows again a cross section ofthe target, that is the target thickness in percent as a function of theradius in mm of the target disk. The new condition of the profiledtarget surface is denoted by E0, El after an operating time of 60 kWhand E2 after an operating time of 121 kWh. In both FIGS. 10 and 12 isevident the manner in which the inventive profiling of the dielectrictarget has a positive effect. The distribution profile T11 to T15 showvery low discrepancies over the entire target working life. This showsdirectly the stabilizing effect of target profiling over the targetworking life. A further advantage of target profiling results therefromthat in particular when high requirements are made of the layerthickness precision on the substrate the material utilization of thetarget is increased since the entire target thickness can be betterutilized. On the one hand, this leads to better material utilizationand, on the other hand, to longer service life of the target andconsequently to higher throughputs per target employed, whichsignificantly increases the economy. Overall also a slightly higher ratecan be observed through the decrease of the target impedance in thezones of reduced target thickness. The process can, moreover, also becarried out more simply since hardly more expenditures are necessarywith respect to the external control or readjustment of the sputteringprocess for attaining a good distribution. Overall the structuring ofthe cathode and the entire configuration is thereby also simplified.

1. Method for sputtering from a dielectric target (9) in a vacuumchamber (2) with a high frequency gas discharge, with the target (9)being mounted on a cooled metallic back plate (10) and this back plateforming an electrode (10) supplied with high frequency, the methodcomprising: the target thickness (Td) being profiled (15) differentlythick over the surface such that in the regions of a desired decrease ofthe sputtering rate the target thickness (Td) is selected to be greaterthan in the remaining regions.
 2. Method as claimed in claim 1, whereinthe high frequency gas discharge is magnetic field-enhanced (11). 3.Method as claimed in claim 2, wherein the magnetic field is movedrelative to the target.
 4. Method as claimed in claim 2, wherein thetarget profiling (15) compensates the erosion profiling (14) of themagnetic field, such that over the target working life the layerthickness distribution follows, which changes over the course of thetarget erosion and through the increasingly strong magnetic fieldeffect.
 5. Method as claimed in claim 1, wherein the target profiling(15) is provided such that a desired layer thickness distribution on thesubstrate occurs.
 6. Method as claimed in claim 1, wherein the targetmaterial is profiled on the front and/or on the back side and, in thecase of back side profiling of the target (9), the back plate (10)follows the target profiling.
 7. Method as claimed in claim 1, whereinbetween the target (9) and the back plate (10) a profiled compensationdielectric (17) in thermal contact is disposed and the back plate (10)essentially follows this profiling.
 8. Method as claimed in claim 1,wherein the high frequency gas discharge is magnetic field-enhanced (11)with a magnetron magnetic field generated by a magnet system (11) beingat least partially disposed in the region of the target back side. 9.Method as claimed in claim 8, wherein the magnetic field is rotatedrelative to the target about a central axis (12).
 10. Sputtering targetof a dielectric material (9) bonded on a support (10) of metal, whereinthe target (10) on a front side and/or on a back side has a profiledstructure (15, 16) and between the dielectric target (9) and theprofiled support, a compensation dielectric (17) is disposed. 11.Sputtering target as claimed in claim 10, wherein in the case of atarget (9) profiled on the back side, the support (10) follows theprofiling (16), with, in particular, the target (9) as well as also thesupport (10) being essentially developed in the form of plates. 12.Sputtering target as claimed in claim 10, wherein the profiling (15, 16)in the regions of the strongest target erosion is developed thicker thanin the remaining regions.
 13. Sputtering target as claimed in claim 10,wherein the compensation dielectric has an of 2 to 50 and preferably iscomprised of ceramics with good thermal conductivity.
 14. Sputteringtarget as claimed in claim 13, wherein the ceramics is Al₂O₃. 15.Sputtering target as claimed in claim 10, wherein the target (9) is amagnetron sputtering target.
 16. High-frequency magnetron sputteringsource (1), containing a target (9) according to claim
 10. 17. Magnetronsputtering source as claimed in claim 16, wherein a high frequencygenerator (20) is connected with the target support (10) and the highfrequency generator (20) generates a frequency ≧1 MHz.
 18. Sputteringtarget of dielectric material (9) bonded on a support (10) comprised ofmetal, comprising the target (9) on a front and/or back side, has aprofile structure (15, 16), with the target comprising ZnS and SiO₂ andthat at 13.5 MHz the value of the loss angle δ satisfies the conditiontan(δ)<0.05.
 19. High-frequency magnetron sputtering source (1),containing the target of claim
 18. 20. Magnetron sputtering source asclaimed in claim 19, wherein a high frequency generator (20) isconnected with the target support (10) and the high frequency generator(20) generates a frequency ≧1 MHz.